One of the important aspects of learning and teaching through cooperation is the group composition or grouping "who with whom". An unresolved issue is that of the superiority of heterogeneity or homogeneity in the structure of the groups. The present study was an attempt to investigate the impact that homogeneous and heterogeneous groupings of Iranian EFL learners regarding their prior levels had on their writing ability when working cooperatively. Having administered a standardized preliminary English test (PET) and a writing test taken from PET sample tests as a pre-test, 66 high and low proficient learners were assigned into three groups: heterogeneous, homogeneous high, and homogeneous low groups. Following the end of the treatment that took 10 sessions each for 30 min, all groups received a writing test as a post-test. The results demonstrated that learners improved their performance through cooperation, whether working with stronger or weaker peers. However, heterogeneous grouping showed superiority over homogeneous grouping at the low level. Low students in the heterogeneous class made more relative gains than high students in the same class. It must be noted that low students did not improve at the expense of high students. The results revealed that cooperative learning could be especially beneficial for low students. It is hoped that the findings of the present study will give teachers deep insights into group compositions in cooperative learning courses, and will help them make better group experiences for students.